Designing Power Protection Systems and UPS Topologies


With increasing power consumption and business dependence on electrical power. It is more important than ever that the topology and design of uninterruptible power supplies (UPS). Meet the needs of applications to ensure the highest level of power protection and business continuity.

An uninterruptible power supply (UPS) is located between the critical load and the source of AC power. Typically the electrical grid. It provides two functions: a secure source of power in the event of a grid failure. And a clean, stable, regulated power supply when the grid is available.

It is this second, lesser-known function that is becoming increasingly important to modern businesses. Especially those that rely heavily on computer and telecommunications technologies. This type of equipment is more susceptible to current and voltage fluctuations and other power supply problems (surges, brownouts, outages, transients, and harmonics) that are. Typically associated with untreated mains power. These disturbances can lead to costly equipment failures and data loss or corruption.

There are three main types of static UPS: inline, offline, and inline interactive. Which differ in their ability to perform these critical functions, as well as in their level of safety and power protection.

Inline and offline interactive UPSs are. Limited in design to small applications such as homes and small offices. In comparison, online UPS systems offer excellent electrical performance, reliability, and fault tolerance. They are suitable for powering a wide range of loads, from mission-critical file servers and telecommunications centers. To complete industrial manufacturing facilities and data centers.

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UPS Sizing


All uninterruptible power supplies contain an inverter that digitally generates an AC waveform from a DC source to power the connected load.

In the online UPS, the inverter is design for continuous operation. The raw power from the mains is a true sine wave, therefore the output waveform of this type of UPS is also a sine wave. This type of inverter is classified as voltage and frequency independent (VFI). The operation of the inverter is not affect by changes in grid voltage or frequency.

In-line or stand-alone interactive inverters contain much less complex inverters that are used only in standby mode and are ready to feed the load in the event of a mains power failure or fluctuations outside the specified input voltage and frequency window. The least complex type of inverter is the one used in pure stand-alone inverters (such as those used in home offices).

The size of the inverter limits the load capacity of the inverter and is measured in VA (inverter output voltage x amps).


Continuous operation of the inverter in an uninterruptible power supply requires a constant source of DC power, which is provided by a rectifier connected to the grid.

Rectifiers can generate high levels of harmonics depending on their design, mode of operation, and type of inverter. In a transformer-based inverter, the rectifier not only powers the inverter but also charges a number of batteries. In the transformerless version, a step-up converter is installed between the rectifier and the inverter to raise the DC voltage of the rectifier (or the battery set) to the higher DC voltage level required by the inverter. The output of the boost converter also charges the battery.

In interactive or stand-alone linear UPS systems, the charger connected to the mains.

Battery packs

The battery in a UPS is rated in watts. It provides DC power to the UPS when the mains supply is interrupted. The amount of time the UPS runs on battery power is called “battery run time”, “runtime” or “standby time”.

A string consists of one or more battery packs connected in series. The operating time of the battery is measure in Ah (ampere-hours) and depends on the power of the battery pack and the size of the connected load. The lower the load, the longer the battery life.


UPS systems contain EMI/RFI (electromagnetic and radio frequency interference) filters that protect them (and the load) from dangerous mains surges and electrically induced interference. To this end, voltage spikes are suppress or “clamped” to an acceptable level.

These filters, which vary in complexity, are use in all three uninterruptible power supply topologies. The filters also prevent electrical impurities that may generated by the high-frequency electronics of the inverter itself from interfering with upstream or downstream equipment.


The main reason that uninterruptible power supplies are the best choice for protecting critical loads is that they have an automatic bypass that provides fail-safe protection in the event of a system failure. This is a “safe bypass”. In the event of a system failure, the bypass transfers the load to another power source (mains power or another power path, e.g. maintenance bypass or centralized static switches operating in parallel UPS configurations). The bypass automatically transfers the load to the UPS output after the fault is clear.

UPS topologies

The in-line or double-conversion UPS keeps voltage and frequency fluctuations within specified limits. So that the output power is not affect by mains fluctuations. Not only does the UPS condition the mains supply to avoid problems, but it is also the only UPS solution that ensures uninterrupted power to the business or data center in the event of a power failure.

The linear-interactive UPS uses integrated passive electronic regulators to stabilize and manage voltage fluctuations. When mains power is available, the inverter output frequency monitors the mains input frequency. Inline and interactive UPSs have a tower or rack enclosure format. And installed near the respective loads, typically in a data center or computer room.

The offline or passive standby UPS is a compact desktop or wall-mounted unit. That uses its own output to monitor the voltage and frequency fluctuations of the mains.


UPS accessories include external bypasses that allow maintenance to performed without disrupting the load. Paralleling kits that can used to achieve a higher level of operational stability, and battery extension units. That provide a simple and convenient way to extend runtime.

Depending on the size and type of load, application, and criticality. A complete UPS solution may include one or more of the three basic topologies. For example, a networked UPS is considering the most advanced solution for power protection. While an in-line UPS is an intermediate solution. The standalone topology provides basic power protection. And well suited for small and limited applications. Such as small offices or homes.

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